A tunnel-making process can use a tunnel excavator which has a working chamber operating under atmospheric pressure in which the local front wall is supported with the help of a pressurized medium.
The working chamber is connected by a controlling gap of a shield cover with the gap between the shield cover tail of the excavator and/or the earth or ground and the tunnel-lining member. The gap between the shield cover tail and the tunnel-lining member is closed off from the working chamber by a gap-sealing ring and concrete is forced into the gap through a feeder pipe in the gap-sealing ring.
Thus it is possible to operate in loose ground. The pressurized medium is a fluid medium. It can be pressurized with water, a thixotropic fluid or with a gas, especially air. The concrete can have any binder, especially a hydraulic binder or a plastic resin binder. The gap-sealing ring can be constructed in different ways.
Permeable gaps between the gap-sealing ring and the shield cover tail and/or the tunnel-lining member cannot always be prevented. That is particularly true when the tunnel-lining member is constructed as a tubing assembled from a plurality of tubing segments and a slight displacement of the individual tubing segments relative to each other in a radial direction cannot be prevented. In particular, this problem has serious consequences as described below.
In tunnel excavation, especially in loose ground, the local front wall is supported either mechanically by a excavating disk or by a pressurized medium. The mechanical support is incomplete and causes deformation of the local front wall which triggers sinking or settling of the upper surface of the ground. Support of the local front wall by a fluid is very effective and leads to tunnel excavation characterized by very little settling of the ground.
It is disadvantageous, however, that the broken earth must be removed mixed with the fluid. It is separated from the fluid below ground which is particularly expensive with fine-grained earth.
Supporting the local front wall with pressurized air is particularly advantageous by contrast, because the excavated earth can be transported away dry.
In current practice, the entire tunnel pipe can be put in place under pressurized air to support the ground at the local front wall. Attempts to put only the working chamber in the front portion of the shield cover under pressurized air to allow the digging team or crew to operate under atmospheric pressure indeed have been known to fail.
Pressurized medium losses and particularly pressurized air losses occur when the pressurized medium flows rearwardly in the controlling gap to the outside of the shield cover and forces its way through the incomplete seal at the gap-sealing ring into the working chamber. This gap which has a thickness of about 10 cm is simultaneously filled with concrete in the described way on forward motion of the shield cover to prevent the surrounding earth which can also be below the water table from entering the gap. It is not guaranteed, however, that the applied concrete pressure is always reliably greater than the pressure which arises because of load. Hence earth can fall into the gap so that it is impossible to fill the gap at the shield cover tail completely. Similarly that condition also is effected when the tunnel is made in ground comprising loose or broken stone.
The pressurized medium, especially a gaseous pressurized medium, which runs through the gap surrounding the shield cover until behind the shield cover tail, flows through an only incompletely filled gap. If the gap-sealing ring is not sealed, the pressurized medium in the working chamber escapes.
An incomplete filling of the gap can be countered with a moving elastically supported gap-sealing ring (see, for example, German Patent document 36 42 893.0-24).
However, a reliable filling of the gap cannot be attained in this way, when a tubing with tubing segments is used for a tunnel-lining member and an offset between adjacent tubing segments can develop over which the seal of the gap-sealing ring passes producing leakage gaps with widths up to 15 mm.
The fluid concrete forced-in flows through this permeable seal into the shield cover interior unless the provided pressure can be maintained in the concrete for support of the of about 10 cm is simultaneously filled with concrete in the described way on forward motion of the shield cover to prevent the surrounding earth which can also be below the water table from entering the gap. It is not guaranteed, however, that the applied concrete pressure is always reliably greater than the pressure which arises because of load. Hence earth can fall into the gap so hat it is impossible to fill the gap at the shield cover tail completely. Similarly that condition also is effected when the tunnel is made in ground comprising loose or broken stone.
The pressurized medium, especially a gaseous pressurized medium, which runs through the gap surrounding the shield cover until behind the shield cover tail, flows through an only incompletely filled gap. If the gap-sealing ring is not sealed, the pressurized medium i the working chamber escapes.
An incomplete filling of the gap can be countered with a moving elastically supported gap-sealing ring (see, for example, German Patent document 36 42 893.0-24).
However, a reliable filling of the gap cannot be attained in this way, when a tubing with tubing segments is used for a tunnel-lining member and an offset between adjacent tubing segments can develop over which the seal of the gap-sealing ring passes producing leakage gaps with widths up to 15 mm.
The fluid concrete forced-in flows through this permeable seal into the shield cover interior unless the provided pressure can be maintained in the concrete for support of the surrounding earth. The danger of the concrete flowing away in this manner is, however, greater when the concrete pressure is higher which is required when the tunnel being dug is deep.